Modern chemical technology and environmental concerns have required the determination of sulfur in a variety of matrices down to levels at or below one part per million (ppm). Polymerization inhibitors, catalyst poisoning, and sulfur emissions are especially important areas for sulfur determination. A similar need exists for the ability to rapidly determine trace sulfur in highly halogenated and/or nitrogen containing feedstocks or products.
Although many methods have been developed for the determination of total sulfur at ppm levels, these methods have often been complicated and time consuming. Further, certain constituents in the sample matrix have made some methods impossible to apply.
Among specific prior art methods, the use of combustion-microcoulometry for the determination of sulfur has been widely documented, and systems are commercially available. Both oxidative and reductive pyrolysis with microcoulometric detection have been applied.
In the case of oxidative pyrolysis, the sulfur in the sample matrix is converted to sulfur dioxide (SO.sub.2) in an oxygen rich atmosphere at 800.degree. to 1000.degree. C. The SO.sub.2 thus formed is trapped in an acidic buffered electrolyte containing a small amount of iodine (I.sub.2). The SO.sub.2 reacts with iodine to form SO.sub.3, while the iodine is reduced to iodide ion (I.sup.-). The iodide ion is coulometrically reoxidized to iodine, the amount of charge required being proportional to the original sulfur content of the sample.
Reductive pyrolysis coupled with microcoulometric detection requires that the sulfur be converted to hydrogen sulfide (H.sub.2 S) in a hydrogen atmosphere at 1100.degree. to 1150.degree. C. The sulfide formed is trapped in a basic electrolyte containing free silver (Ag.sup.+) ions. The sulfide entering the cell reduces the silver ion concentration by precipitation of silver sulfide (Ag.sub.2 S), and the silver ion concentration is reestablished coulometrically.
Both the described coulometric methods are nonspecific, and thus suffer from several interferences. For oxidative pyrolysis, chlorine (Cl.sub.2) or bromine (Br.sub.2) produced from combustion of halogenated organic materials can replace iodine in the titration step, resulting in an artificially low sulfur assay for the injected sample. Iodine may also be blown out of the titration cell by the pyrolysis gases. Similarly, aldehydes and nitrogen (as NO) can be titrated with iodine to give a positive error. For reductive pyrolysis, halide ions or nitrogen (as HCN) formed during the combustion will precipitate silver ion, giving an artificially high sulfur measurement. Although the solubility product of silver sulfide is much lower than that of silver halides (10.sup.-50 vs. 10.sup.-11 for AgCl) favoring silver sulfide precipitation, halogen concentrations in excess of ten percent can introduce a significant error in these measurements.
An alternative prior art method for determination of sulfur in halogenated matrices is the reduction of sulfur-containing compounds to sulfide using Raney nickel catalyst. The sulfide can be determined colorimetrically by reaction with methylene blue, or by potentiometric methods. Although both excellent accuracy and precision have been reported, the method requires several hours per determination and sulfur recoveries may vary for different matrices, as is the case for various isomers of dichlorobenzene.
In still another reductive pyrolysis technique recently reported, Anal. Chem., 50, 76 (1978), sulfur compounds are noncatalytically reduced under hydrogen pyrolysis conditions to H.sub.2 S, which is monitored by photometric measurement of the blackening of a moistened lead acetate-impregnated paper tape. The method is rapid and sensitive but is nonstoichiometric for several common sulfur compounds.
In respect to yet other of the diverse methods developed for measuring sulfur, chemical reduction methods have been proposed, e.g., based on hydriodic acid reduction of sulfur to sulfide with plasma emission detection. These methods are generally satisfactory for inorganic sulfur species, but recoveries are substantially lower for organic species. Replacement of the hydriodic acid reduction apparatus with a reductive pyrolysis system would increase recovery for organic species, but potential hazards due to ignition of hydrogen in the plasma could not be avoided.
Combustion methods, e.g., oxidation flask techniques have also been developed using either x-ray fluorescence detection, thermal conductivity, ion chromatography, potentiometry, polarography or gravimetry for detection. Generally, these methods, however, are applicable only to relatively high sulfur concentrations (i.e., &gt;50 ppm S).
Yet, still other methods which are more sulfur-species specific than the pyrolysis methods discussed, such as those based on gas chromatography with electrolytic conductivity as detection, are more applicable to those matrices which are highly volatile, and thus are restrictively limited in scope of utility.
Accordingly, it is an objective of this invention to provide an improved and broadly applicable method for determining trace sulfur, and which method may be particularly advantageously applied to determine total organic sulfur in diverse matrices.
It is particularly an objective hereof to provide such method which offers all of the combined and important advantages of specific sulfur detection, rapid analysis times, low detection limits, freedom from halogen and nitrogen interferences and solvent effects, and minimal operator attention.